On site application of paint directly to a surface to be decorated is the time-honored method for providing a graphic design such as a decorative design. While this provides many aesthetic and physical features including realistic appearance, color flexibility, and durability to abrasion, weathering, and chemical attack, it also suffers from many disadvantages. Such disadvantages include the need for relatively skilled labor, long application times, and potential contamination to adjacent areas (particularly mechanical equipment). Accordingly, prefabricated film graphics have been utilized to avoid many of these disadvantages. Such graphics can be manufactured at a convenient location and subsequently applied on site by relatively unskilled labor efficiently with virtually no threat of contamination or health hazard.
One type of prefabricated graphic comprises sheets of polymeric film bearing a graphic design and a non-registered layer of adhesive under the graphic design which is protected by a liner. The sheets are die and/or "kiss cut" to provide the desired design. The design is bonded to the desired substrate by means of the adhesive layer after removal of the liner. Such graphics are generally limited to relatively simple configurations. Furthermore, accurate die and/or kiss cutting is difficult to achieve. Moreover, there is a substantial amount of material waste inherent with this method.
While die and/or kiss cutting does serve to provide a dry transfer article in which the adhesive is in registry with the graphic design, registry may also be accomplished by exact registration of the adhesive applied to the graphic design. U.S. Pat. Nos. 4,028,474 and 4,028,165 exemplify this approach. There is general recognition, however, that it is difficult and sometimes costly to achieve satisfactory alignment of adhesive and graphic, especially for intricate patterns.
Other approaches are known for aligning adhesive with the graphic design. For example, in U.S. Pat. No. 3,684,544, the adhesive is initially covered with a continuous silica coating which interferes with bonding the article to a substrate. Silica is displaced by adhesive in the regions underlying the ink design as a result of altering the adhesive rheology by applying pressure to the article causing release of a fluidity agent from the ink.
U.S. Pat. No. 4,286,008 discloses an article in which a photopolymerizable ink is screen printed onto a carrier film. In one embodiment, an article is provided with an adhesive layer overlapping the ink layer. Cleavage of the adhesive layer along the edge of the ink is taught to occur.
U.S. Pat. No. 3,987,225 discloses an article of the type having a continuous adhesive layer. The adhesive is edge stressed by incorporating a solvent or dispersing powder in the adhesive. This is asserted to permit the adhesive to shear cleanly along the edge of the design, eliminating residual particles, or strings of adhesive.
U.S. Pat. No. 4,288,525 discloses a peel-apart dry transfer material in the form of opposing support layers. A continuous photosensitive layer and a continuous image-forming layer is sandwiched between the opposing support layers. The image-forming layer may contain an adhesive component, or a separate adhesive layer may be provided between the image-forming layer and an adjacent carrier film, or the adhesive may be applied after the exposure and peel-apart development. One use of the dry transfer material is to place the developed structure on a rigid, transparent support, adhesive-side down, and irradiate through the support to provide a strong bond such that the carrier film can then be stripped away leaving the photosensitive layer, the image layer, and the adhesive bonded to the substrate.
United Kingdom Pat. No. 2,053,497 discloses a peel-apart laminate composed of opposing carrier sheets which are transparent or translucent, between which are sandwiched a continuous image-forming layer or at least one further layer which is photosensitive, and an adhesive layer. After imagewise exposure to actinic radiation through a transparency, the laminate is peeled apart providing two decals or signs, one a positive and the other a negative of the original. Either is applied adhesive-side down to a substrate with the carrier sheet providing a protective covering for the underlying image layer.
The aforementioned patents all describe approaches which rely upon mechanisms which are difficult to control or which, in some cases, dictate use of materials unable to withstand demanding environments to achieve clean separation/development of the transfer material elements. In other instances, the development mechanism dictates use of a single color graphic, a severe limitation to general use of the article. Further, in the case of prior photosensitive dry transfer articles, there is dependence upon the use of an external mask to provide the necessary exposure pattern of actinic radiation for creation of the latent image.
Yet another approach is provided in U.S. Pat. No. 4,454,179 which discloses an article having a continuous, actinic radation-transparent support, a continuous layer of an actinic radiation responsive adhesive and a graphic design interposed between the adhesive and the support. The graphic design is applied in a predetermined manner and is opaque to actinic radiation so that after the article has been exposed to such radiation through the support and applied to a substrate, removal of the support allows selective separation of the support and the radiation exposed portions of the adhesive from the substrate along the edges of the graphic design. The design and the underlying unexposed portions of the adhesive remain on the substrate. This patent describes only articles made in reverse order, that is the final color is the first applied to the support, after which the adhesive layer is applied. The use of reverse order manufacture can make color matching difficult when more than one color is to be employed in the graphic design. Additionally, this patent describes the direct application of the graphic to the support. Generally, only low adhesive forces hold the graphic to the support. This can lead to difficulty in positioning, or repositioning, the graphic on the substrate, as the graphic may release prematurely from the support.
In addition to the foregoing, carriers which are not transmissive to actinic radiation, such as paper, cannot be used with the construction immediately discussed above.
Furthermore, inks, which are conventionally used as imaging materials to prepare the graphic design, which are translucent or transparent to actinic radiation cannot be utilized therewith. In this instance, the adhesive characteristics beneath the graphic design are obviously very dependent on the effectiveness of the graphic to mask the actinic radiation. However, many colors, such as tints, translucent metallics and pearlescents are transmissive to actinic radiation, thus resulting in high adhesion loss of the graphic to the adhesive. Such a result hampers the application of the graphic because the adhesion of the second adhesive layer to the substrate is greatly reduced.
In addition, a system based on crosslinking the adhesive with actinic radiation in actually creates an adhesive with increased film properties. As a result, the force necessary to fracture the adhesive at the edge portion of the graphics is increased and results in a converse decrease in ease of application.
Further, actinic radiation-responsive adhesives are not stable to storage at elevated temperatures (i.e., 125.degree. F.) for periods greater than about two weeks. If this occurs, loss of adhesion of the adhesive to the substrate occurs, resulting in poor application characteristics.